(Repeats Sept. 20 story for wider distribution.) By Svea Herbst-Bayliss BOSTON, Sept 20 (Reuters) - Billionaire hedge fund manager William Ackman, whose firm owns an 8.3 percent stake in Automatic Data Processing Inc, is turning up the heat in his proxy war for the human resources outsourcing company by targeting an often-ignored but influential group of people - retail investors...